Following the i.c. inoculation of dengue type 2 virus (DV) the spleen weight of infected mice was reduced, as was the proportion of cells killed by ATS and complement (T lymphocytes) in spleen-cell suspensions. In DV-infected mice the mean haemolysin titre, 16 days after i.p. inoculation of 4 x 10(8) SRBC, was 47 compared with 406 in normal mice and spleen cells from DV-infected mice produced significantly reduced direct GVH reactivity in Parker strain (PS) infant mice. Adoptive transfer of spleen cells obtained from mice given three weeks i.p. doses of DV or a single i.c. dose, suppressed antigen-specific antibody secretion as detected by Jerne plaque technique. This suppression was abrogated by pretreating the transferred cells with ATS and complement. Thus DV selectively depletes T-lymphocyte subpopulations responsible for helper and effector functions and spares suppressor T cells in the spleen of infected mice.